The three-dimensional isothermal flow fields of single screws with different values of tip-clearance (0.4, 0.8, 1.2 mm)were simulated by POLYFLOW software. Combined with the particle tracing technique, the effect of different tip- clearance on the flow field and mixing ability of the food melt was also explored. The results show that: the shear rate near the top of the screw flight increases, the average viscosity of food melt decreases, and the conveying capacity of the screw weakens with the decrease of the tip-clearance. On the contrary, with the increase of the tip-clearance, the residence time of the food melt increases first, then decreases subsequently, and the dispersion mixing effect becomes poorer. When the tip-clearance increases to value of 1.2 mm, the residence time of the food melt is even longer than the one with the value of 0.4 mm. It also shows that if the tip-clearance is too large, the residence time of the food material might become hard to be controlled and the mixing effect of these food material components would also become worse. As a result, the tip-clearance between screw and barrel could significantly influence both the flow field and mixing characteristics of food materials, when the tip-clearance is 0.8 mm, it can not only ensure the transport capacity to the food melt, but also behave the good flow field distribution and mixing effect. Therefore, it is the optimal tip-clearance value deduced by the numerical simulation.